When printing with a printer, the register and gage-pin must be monitored and calibrated. In particularly, the gage-pin can be calibrated in an electrophotographic printer by the positioning description of a photoconductor.
To control the gage-pin, color register marks are normally printed on a print substrate or transparent transport band for the print substrate, and by the individual printing jobs in order to print the different color printouts, preferably in cyan, magenta, yellow and black for a multi-color printout for example. A sensor allows the positions of these color register marks to be set relative to each other. To do this, the sensor emits light and receives light, which is reflected back into the sensor by the area around each color register mark, namely by the print substrate or a reflector arranged on the other side of the transport band, which is transparent for the light. Here, the absorption of the light in the area of the color register mark is itself greater than in its own vicinity, so that a contrast is created, which allows the sensor to recognize at least one edge of the color register mark in the working field of the sensor.
To check the register, an edge of a print substrate must be recognized, onto which the printed image is supposed to be printed on the print substrate. Preferably, the front edge of a printed sheet in the sheet transport direction will be ascertained by doing this. Normally, a light barrier is used for this, where a light emitter is arranged on one side of the transparent transport band for the print substrate, preferably underneath the transport band and light is sent through the transport band to a light receiver on the other side of the transport band. This light path is first partially and then completely broken by the arrival of a printed sheet, whereby the edge of the printed sheet can be localized within the working area of the light barrier.